Asymmetric conjugate addition of grignard reagents to pyranones

Article abstract of DOI:10.1021/ol303141x

An efficient enantioselective synthesis of lactones was developed based on the catalytic asymmetric conjugate addition (ACA) of alkyl Grignard reagents to pyranones. The use of 2H-pyran-2-one for the first time in the ACA with Grignard reagents allows for a variety of further transformations to access highly versatile building blocks such as β-alkyl substituted aldehydes or β-bromo-γ-alkyl substituted alcohols with excellent regio-and stereoselectivity.

Full text of DOI:10.1021/ol303141x

ORGANIC
LETTERS
2
013
Vol. 15, No. 2
86–289
Asymmetric Conjugate Addition of
Grignard Reagents to Pyranones
2
Bin Mao, Mart ´ı n Fa ~n an ꢀa s-Mastral, and Ben L. Feringa*
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4,
9
747 AG Groningen, The Netherlands
b.l.feringa@rug.nl
Received November 14, 2012
ABSTRACT
An efficient enantioselective synthesis of lactones was developed based on the catalytic asymmetric conjugate addition (ACA) of alkyl Grignard reagents
to pyranones. The use of 2H-pyran-2-one for the first time in the ACA with Grignard reagents allows for a variety of further transformations to access
highly versatile building blocks such as β-alkyl substituted aldehydes or β-bromo-γ-alkyl substituted alcohols with excellent regio- and stereoselectivity.
3,4-Dihydropyran-2-ones and their derivatives have at-
tracted major attention due to the interesting biological
A concise route for the enantioselective synthesis of enol
lactones through Michael addition followed by cyclization
using sequential catalytic conditions has been described by
1
2
activities and the synthetic perspective associated with
these privileged structures. In recent years, a number of
approaches have been reported toward the preparation of
3
a
Kanemasa et al. Moreover, chiral N-heterocyclic car-
bene (NHC) catalyzed oxo-diene DielsꢀAlder reactions of
R-chloroaldehydes have been successfully applied by the
group of Bode to afford the trisubstituted dihydropyran-2-
3
chiral 3,4-dihydropyran-2-ones.
3
b,c
ones in a stereoselective manner.
Mukaiyama and co-
(
1) For representative biological activities of dihydropyran-2-ones
and their derivatives, see: (a) Liu, Z.; Meinwald, J. J. Org. Chem. 1996,
1, 6693. (b) Hagen, S. E.; Vara Prasad, J. V. N.; Boyer, F. E.;
3
d,e
workers reportedthe organocatalyticconversion of silyl
enolates and R,β-unsaturated ketones into the optically
active 3,4-dihydropyran-2-ones employing cinchona alka-
loid derived chiral quaternary ammonium phenoxides.
Slower to develop, however, have been protocols that allow
the introduction of alkyl groups at the newly formed stereogenic
center in the structurally diverse 3,4-dihydropyran-2-ones.
While addressing this challenge, we envisioned the possibility
6
Domagala, J. M.; Ellsworth, E. L.; Gjda, C.; Hamilton, H. W.;
Markkoski, L. J.; Steinbangh, B. A.; Tit, B. D.; Lunney, E. A.;
Tummino, P. J.; Fergnson, D.; Hupe, D.; Nouhan, C.; Gracheck,
S. J.; Saunders, J. M.; Vander Roest, S. J. Med. Chem. 1997, 40, 3707.
(c) Douglas, C. J.; Sklenicka, H. M.; Shen, H. C.; Mathias, D. S.; Degen,
S. J.; Golding, G. M.; Morgan, C. D.; Shih, R. A.; Mueller, K. L.; Scurer,
L. M.; Johnson, E. W.; Hsung, R. P. Tetrahedron 1999, 55, 13683. (d)
Yamamoto, H.; Katano, N.; Ooi, A.; Inoue, K. Phytochemistry 2000, 53,
7. (e) Evidente, A.; Cabras, A.; Maddau, L.; Serra, S.; Andolfi, A.;
Motta, A. J. Agric. Biol. Chem. 2003, 51, 6957. (f) Fairlamb, I. J. S.;
Marrison, L. R.; Dickinson, J. M.; Lu, F. J.; Schmidt, J. P. Bioorg. Med.
Chem. 2004, 12, 4285.
4
of applying 2H-pyran-2-ones together with alkyl Grignard
reagents to access optically active dihydropyran-2-ones via
(2) For selected examples of the synthetic application, see: (a) Schultz,
A. G.; Pettus, L. J. Org. Chem. 1997, 62, 6855. (b) Zhou, X.; Wu, W.; Liu,
X.; Lee, C.-S. Org. Lett. 2008, 10, 5525. (c) Willot, M.; Radtke, L.; Konning,
D.; Frohlich, R.; Gessner, V. H.; Strohmann, C.; Christmann, M. Angew.
Chem., Int. Ed. 2009, 48, 9105. (d) English, B. J.; Williams, R. M. J. Org.
Chem. 2010, 75, 7869. (e) Lehr, K.; Mariz, R.; Leseurre, L.; Gabor, B.;
Furstner, A. Angew. Chem., Int. Ed. 2011, 50, 11373.
(4) For reviews on 2H-pyran-2-ones, see: (a) Afarinkia, K.; Vinader,
V.; Nelson, T. D.; Posner, G. H. Tetrahedron 1992, 48, 9111. (b)
McGlacken, G. P.; Fairlamb, I. J. S. Nat. Prod. Rep. 2005, 22, 369.
(5) For reviews on the copper-catalyzed asymmetric conjugate addi-
tion, see: (a) Feringa, B. L. Acc. Chem. Res. 2000, 33, 346. (b) L oꢀ pez, F.;
Minnaard, A. J.; Feringa, B. L. Acc. Chem. Res. 2007, 40, 179. (c)
Harutyunyan, S. R.; den Hartog, T.; Geurts, K.; Minnaard, A. J.;
Feringa, B. L. Chem. Rev. 2008, 108, 2824. (d) Alexakis, A.; B €a ckvall,
J. E.; Krause, N.; P ꢁa mies, O.; Di ꢀe guez, M. Chem. Rev. 2008, 108, 2796.
(e) Jerphagnon, T.; Pizzuti, M. G.; Minnaard, A. J.; Feringa, B. L. Chem.
Soc. Rev. 2009, 38, 1039. (f) Wang, S. Y.; Loh, T. P. Chem. Commun.
2010, 46, 8694.
(3) For representative examples of the asymmetric synthesis of dihy-
dropyran-2-ones, see: (a) Itoh, K.; Hasegawa, M.; Tanaka, J.; Kanemasa,
S. Org. Lett. 2005, 7, 979. (b) He, M.; Uc, G. J.; Bode, J. W. J. Am. Chem.
Soc. 2006, 128, 15088. (c) Kaeobamrung, J.; Kozlowski, M. C.; Bode, J. W.
Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 20661. (d) Tozawa, T.; Yamane, Y.;
Mukaiyama, T. Chem. Lett. 2006, 35, 56. (e) Tozawa, T.; Nagao, H.;
Yamane, Y.; Mukaiyama, T. Chem. Asia J. 2007, 2, 123.
1
0.1021/ol303141x r 2013 American Chemical Society
Published on Web 01/07/2013

the copper-catalyzed asymmetric conjugate addition (ACA)
5
reaction. 2H-Pyran-2-ones featuring electron deficient
addition (ACA) of Grignard reagents to acyclic R,β-
unsaturated compounds. The copper-catalyzed ACA
5,8
diene moieties are common precursors for [4 þ 2] and
reaction of Grignard reagents to cyclic enones has also been
successfully achieved with excellent enantioselectivities by
6
[
However, the increased electron delocalization, compared
2 þ 2] cycloadditions to construct bicyclic building blocks.
9
several groups. However, despite wide scope, this method
still presents limitations using less reactive substrates such as
with acyclic R,β-unsaturated esters, results in a lower reac-
tivity toward ACA reactions. To the best of our knowledge,
7
simple cyclic R,β-unsaturated esters, providing lactones only
with moderate enantioselectivity.
9a,e
asymmetric 1,4-addition of Grignard reagents to 2H-pyran-
2-ones still remain elusive. Such a reaction could provide an
efficient, direct, and versatile method toward the synthesis of
chiral 3,4-dihydropyran-2-ones. In addition, the resulting
chiral intermediates would allow for a variety of further
transformations in particular by addition to the enol ester
moiety to access highly versatile intermediates with excellent
regio- and stereochemical control (Scheme 1).
Figure 1. Chiral diphosphine ligands for copper-catalyzed
asymmetric conjugate addition.
Scheme 1. Catalytic Asymmetric 1,4-Addition and Proposed
Routes to Chiral Intermediates by Further Transformation of
Dihydropyran-2-ones
Inspired by our recent study of the Cu-catalyzed ACA
1
0
reaction of Grignard reagents to coumarins, we present
here the Cu-catalyzed conjugate addition of Grignard
reagents to 2H-pyran-2-ones by using ferrocenyl-based
bisphosphine ligands (Figure 1), providing the regio- and
enantioselective synthesis of 3,4-dihydropyran-2-ones.
We began our studies by examining the conjugate addi-
tion of ethylmagnesium bromide to 2H-pyran-2-one in the
presence of CuBr SMe and (R ,S)-Josiphos L1 at
Extensive efforts have been made to develop efficient
catalysts for the copper-catalyzed asymmetric conjugate
3
2
Fe
ꢀ
80 °C in CH Cl (Table 1, entry 1). Although potential
2
2
competing pathways for 2H-pyran-2-one include 1,6-
7
(
6) For selected synthetic applications of [4 þ 2] cycloaddition of
H-pyran-2-ones, see: (a) Nicolaou, K. C.; Liu, J. J.; Hwang, C.-K.; Dai,
W.-M.; Guy, R. K. J. Chem. Soc., Chem. Commun. 1992, 1118.
b) Nicolaou, K. C.; Yang, Z.; Liu, J. J.; Ueno, H.; Nantermet, P. G.;
,11
conjugate addition
and 1,2-addition, exclusive forma-
2
tion of 1,4-addition product with moderate enantioselectiv-
ity was observed in the preliminary experiments. When we
turned our attention to (R ,R)-Taniaphos ligand L2, which
(
Guy, R. K.; Clalborne, C. F.; Renaud, J.; Couladouros, E. A.;
Paulvannan, K.; Sorensen, E. J. Nature 1994, 367, 630. (c) Okamura,
H.; Shimizu, H.; Nakamura, Y.; Iwagawa, T.; Nakatani, M. Tetrahe-
dron Lett. 2000, 41, 4147. (d) Shimizu, H.; Okamura, H.; Iwagawa, T.;
Nakatani, M. Tetrahedron 2001, 57, 1903. (e) Baran, P. S.; Burns, N. Z.
J. Am. Chem. Soc. 2006, 128, 3908. (f) Wang, Y.; Li, H. M.; Wang, Y. Q.;
Liu, Y.; Foxman, B. M.; Deng, L. J. Am. Chem. Soc. 2007, 129, 6364.
Fe
has been successfully employed in the conjugate addition of
9
a
Grignard reagents to cyclic enones, 2a was obtained only as
a racemic product. Additionally, ligand L3 ((S)-Tol-Binap)
also provided 2a with low level of enantioselectivity (55:45 er).
In contrast, a promising enantiomeric ratio (86:14 er) was
obtainedwithfullconversion using commercially available
(g) Nelson, H. M.; Murakami, K.; Virgil, S. C.; Stoltz, B. M. Angew.
Chem., Int. Ed. 2011, 50, 3688. For selected [2 þ 2] cycloaddition of 2H-
pyran-2-ones, see: (h) Corey, E. J.; Streith, J. J. Am. Chem. Soc. 1964, 86,
950. (i) Arnold, B. R.; Brown, C. E.; Lusztyk, J. J. Am. Chem. Soc. 1993,
1
2
115, 1576.
reverse-Josiphos ligand L4 (Table 1, entry 4).
(7) For a review on the conjugate addition reactions to electron-
deficient dienes, see: Csaky, A. G.; de la Herran, G.; Murcia, M. C.
Chem. Soc. Rev. 2010, 39, 4080.
Encouraged by the initial screening, general experimen-
tal parameters including solvent and temperature were
examined with respect to yield and regio- and enantios-
electivity. It is important to note that the use of t-BuOMe
as solvent is essential to yield the product in a highly
enantioselective fashion. A slight increase in temperature
(
8) (a) L oꢀ pez, F.; Harutyunyan, S. R.; Minnaard, A. J.; Feringa, B. L.
J. Am. Chem. Soc. 2004, 126, 12784. (b) Pineschi, M.; Del Moro, F.;
Gini, F.; Minnaard, A. J.; Feringa, B. L. Chem. Commun. 2004, 1244. (c)
L oꢀ pez, F.; Harutyunyan, S. R.; Meetsma, A.; Minnaard, A. J.; Feringa,
B. L. Angew. Chem., Int. Ed. 2005, 44, 2752. (d) Mazery, R. D.; Pullez,
M.; Lopez, F.; Harutyunyan, S. R.; Minnaard, A. J.; Feringa, B. L.
J. Am. Chem. Soc. 2005, 127, 9966. (e) Howell, G. P.; Fletcher, S. P.;
Geurts, K.; ter Horst, B.; Feringa, B. L. J. Am. Chem. Soc. 2006, 128,
(ꢀ72 °C) led to full conversion and an enhancement of
enantiomeric ratio to 95:5 (Table 1, entry 8). Different
copper(I) salts in combination with ligand L4 were also
14977. (f) Wang, S.-Y.; Ji, S.-J.; Loh, T.-P. J. Am. Chem. Soc. 2006, 129,
276. (g) Bos, P. H.; Minnaard, A. J.; Feringa, B. L. Org. Lett. 2008, 10,
4219.
(
9) For selected examples of copper-catalyzed ACA reaction of
Grignard reagents to cyclic enones, see: (a) Feringa, B. L.; Badorrey,
R.; Pe n~ a, D.; Harutyunyan, S. R.; Minnaard, A. J. Proc. Natl. Acad. Sci.
U.S.A. 2004, 101, 5834. (b) Martin, D.; Kehrli, S.; d’Augustin, M.;
Clavier, H.; Mauduit, M.; Alexakis, A. J. Am. Chem. Soc. 2006, 128,
416. (c) Matsumoto, Y.; Yamada, K.-I.; Tomioka, K. J. Org. Chem.
008, 73, 4578. (d) Robert, T.; Velder, J.; Schmalz, H.-G. Angew. Chem.,
(10) Teichert, J. F.; Feringa, B. L. Chem. Commun. 2011, 47, 2679.
(11) For selected examples of asymmetric copper-catalyzed 1,6-con-
jugate addition, see: (a) den Hartog, T.; Harutyunyan, S. R.; Font, D.;
Minnaard, A. J.; Feringa, B. L. Angew. Chem., Int. Ed. 2008, 47, 398.
(b) H ꢀe non, H.; Mauduit, M.; Alexakis, A. Angew. Chem., Int. Ed. 2008,
47, 9122.
8
2
Int. Ed. 2008, 47, 7718. (e) Naeemi, Q.; Robert, T.; Kranz, D. P.; Velder,
J.; Schmalz, H.-G. Tetrahedron: Asymmetry 2011, 22, 887.
(12) The opposite enantiomer of ligand L4 is also commercially
available.
Org. Lett., Vol. 15, No. 2, 2013
287

With an effective method for Cu-catalyzed conjugate
addition to 2H-pyran-2-one in hand, we turned our atten-
tion to explore the reactivity of the chiral magnesium
enolate 3, which could be further converted in situ to more
a
Table 1. Screening of Reaction Parameters
8f,10,14
advanced chiral building blocks (Scheme 3).
2H-
Pyran-2-one 1 was treated with hexylmagnesium bromide
under the optimized conditions, followed by quenching the
resulting enolate 3 with MeOH (5 equiv) at ꢀ72 °C. After
warming to room temperature, β-substituted aldehyde 4,
bearing an ester group, was isolated in 66% yield and 94:6 er.
The formation of product 4 is presumably due to protona-
tion of intermediate 3 followed by transesterification. It
should be noted that it is highly challenging to achieve
β-substituted aldehydes by performing the direct asymmetric
conjugate addition with organometallic reagents to sub-
strates such as R,β-unsaturated aldehydes or β,γ-unsatu-
b
c
entry
ligand
solvent
temp (°C)
yield (%)
er
d
1
L1
L2
L3
L4
L4
L4
L4
L4
CH
CH
CH
CH
CH
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
2
ꢀ80
ꢀ80
ꢀ80
ꢀ80
ꢀ80
ꢀ80
ꢀ80
ꢀ72
54
36
40
63
65
69:31
50:50
55:45
86:14
88:12
71:29
92:8
d
2
d
3
d
4
5
f
e
6
Et
2
O
nd
g
38
7
8
t-BuOMe
t-BuOMe
67
95:5
15
rated-R-ketoesters. The present transformation reveals a
valuable alternative upon which to design the stereoselective
synthesis of highly functionalized β-substituted aldehydes in a
concise manner.
a
Formation of 1,6- or 1,2-addition product was not observed unless
2
otherwise noted. General conditions for ACA: 5.0 mol % of CuBr SMe ,
3
b
.0 mol % of ligand, 2 equiv of RMgBr in 7 mL solvent. Isolated yield.
6
c
d
e
2 2
Determined by chiral HPLC analysis. 2.5 mL of CH Cl . nd = not
f
g
determined. 1,2-Addition product was obtained. Full conversion was not
achieved.
investigated. It turned out that the use of a copper halide
was essential in terms of conversion (see the Supporting
Information, Table S1). As reflected by this screening,
CuBr SMe was still the most effective copper(I) salt for
Scheme 3. Trapping/Ring-Opening Reaction of Magnesium
Enolate
3
2
further studies (Table 1, entry 8).
With the optimized reaction conditions established, we
then directed our efforts toward expanding the scope of
Grignard reagents. By employing linear hexylmagnesium
reagent as nucleophile, product 2b was obtained in 82%
yield with 94:6 er (Scheme 2). In addition, the reaction
could be carried out at 3.0 mmol scale to afford the similar
result. Functionalized Grignard reagents bearing a termi-
nal alkene moiety also work well to afford the correspond-
ing product 2c in 95:5 er. Moreover, addition of branched
Grignard reagent (R = i-Bu) provided the desired product
Although strategies have been introduced for the asym-
1
3
metric R-bromination of aldehydes based upon organo-
in good enantiomeric ratio (95:5 er) (2d, Scheme 2).
1
6
catalysis, to date, there is no example for the synthesis of
optically enriched R-bromoaldehyde through a cascade
protocol involving organometallic reagents. Here we envi-
sioned that the resulted 3,4-dihydro-pyran-2-ones 2 are
potential synthons to produce R,β-substituted aldehydes
through diastereoselective R-bromination. Enantioen-
riched product 2b was first treated with NBS for the
a
Scheme 2. Screening of Grignard Reagents
1
7
bromination catalyzed by NH OAc in MeOH to afford
4
(
14) For the synthetic utility of magnesium enolates, see:
ꢂ
Gale ꢂs tokov ꢀa , Z.; Sebesta, R. Eur. J. Org. Chem. 2011, 2011, 7092.
15) (a) Fa n~ an ꢀa s-Mastral, M.; Feringa, B. L. J. Am. Chem. Soc. 2010,
32, 13152. (b) Palais, L.; Babel, L.; Quintard, A.; Belot, S. b.; Alexakis,
(
1
A. Org. Lett. 2010, 12, 1988. (c) Gremaud, L.; Alexakis, A. Angew.
Chem., Int. Ed. 2012, 51, 794.
(
16) For reviews on the enantioselective R-bromination, see: (a)
a
General conditions for ACA: 5.0 mol % of CuBr SMe
2
, 6.0 mol %
Marigo, M.; Jørgensen, K. A. Chem. Commun. 2006, 2001. (b) Ueda, M.;
Kano, T.; Maruoka, K. Org. Biomol. Chem. 2009, 7, 2005. For
representative examples, see: (c) Bertelsen, S.; Halland, N.; Bachmann,
S.; Marigo, M.; Braunton, A.; Jorgensen, K. A. Chem. Commun. 2005,
3
of L4, 2 equiv of RMgBr in 7 mL t-BuOMe at ꢀ72 °C. Isolated yields.
Enantiomeric ratio of product 2 was determined by chiral HPLC
analysis.
4
821. (d) Kano, T.; Shirozu, F.; Maruoka, K. Chem. Commun. 2010, 46,
7
590.
(
17) Tanemura, K.; Suzuki, T.; Nishida, Y.; Satsumabayashi, K.;
Horaguchi, T. Chem. Commun. 2004, 470.
(
13) A decrease in yield was attributed to the volatility of product.
2
88
Org. Lett., Vol. 15, No. 2, 2013

Scheme 4. R-Bromination of 2b Followed by Reduction
Scheme 5. Copper-Catalyzed ACA of Grignard Reagents to
5
a
,6-Dihydro-2H-pyran-2-one
R-bromoaldehyde 5 in a completely diastereoselective
1
6c
fashion (Scheme 4). Because of instability problems,
the intermediate 5 was then instantly treated with NaBH4
3 equiv) at 0 °C to provide the corresponding alcohol 6 in
1% yield and high enantiomeric ratio (93:7 er). The
relative configuration of product 6 was assigned on the
1
basis of H NMR and NOESY analysis. The stereochemi-
cal outcome is attributed to the stereoinduction of new
stereogenic carbon center formed in the copper-catalyzed
ACA reaction.
(
7
a
General conditions for ACA: 5.0 mol % of CuBr SMe
3
2
, 6.0 mol %
b
of L4, 1.5 equiv of RMgBr in 7 mL of t-BuOMe at ꢀ72 °C. Isolated
c
yield. Enantiomeric ratios of products were determined by chiral GC
analysis.
product 8dwas obtainedasa single enantiomer (>99:1 er).
The copper-catalyzed ACA of less reactive methylmagne-
sium bromide proceeded also, although this transforma-
tion showed lower enantioselectivity (Scheme 5).
In summary, we have developed a highly efficient en-
antioselective synthesis of structurally diverse lactones
based on the asymmetric conjugate addition of Grignard
reagents to 2H-pyran-2-one and 5,6-dihydro-2H-pyran-2-
one. The synthetic applicability of the products from this
catalytic CꢀC bond formation is demonstrated in the
stereoselective synthesis of an optically active β-alkyl sub-
stituted aldehyde and a β-bromo-γ-alkyl substituted alcohol,
which are valuable chiral multifunctional synthons.
We decided also to examine the application of our
catalytic protocol to the copper-catalyzed ACA of Grignard
reagents to 5,6-dihydro-2H-pyran-2-one (Scheme 5).
Although examples of this conjugate addition have been
9a,e,18,19
18
reported, high loading of chiral ligands or reduced
enantioselectivity
protocol. To our delight, the present catalytic system of
9a,e
frequently imposed limitation on this
reversed-Josiphos L4/ CuBr SMe is highly versatile in the
2
3
ACA reaction of Grignard reagents to yield products 8 with
excellent enantioselectivity (up to >99:1 er) (Scheme 5).
Representative results with various Grignard reagents are
summarized in Scheme 5. When substrate 7 was subjected
to ACA conditions using EtMgBr identical to those used
for reactions of 2H-pyran-2-one 1, the desired 1,4-addition
product 8a was obtained with high yield (81%) and
excellent enantiomeric ratio (95:5 er). Noteworthy, the
relative amount of Grignard reagents could be reduced
from 2.0 to 1.5 equiv, possibly due to the higher reactivity
of lactone 7. Transformations with Grignard reagents
bearing longer alkyl chains proceeded to afford the optically
active esters 8b and 8c in 88% and 85% yield with 96:4 and
Acknowledgment. Financial support from The Nether-
lands Organization for Scientific Research (NWOꢀCW)
and Dutch National Research School Combination Catal-
ysis Controlled by Chemical Design (NRSC-Catalysis) is
gratefully acknowledged. B.M. thanks the China Scholar-
ship Council for financial support (No. 2008618001). We
thank M. Smith (GC and HPLC) and T. D. Tiemersma-
Wegman (HRMS) (University of Groningen) for technical
assistance.
97:3 er, respectively (Scheme 5). Remarkably, in the case of
the branched Grignard reagent (R = i-Bu), the desired
Supporting Information Available. Detailed experimen-
tal procedures and full compound characterization data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(
(
18) Kanai, M.; Tomioka, K. Tetrahedron Lett. 1995, 36, 24275.
19) For the copper-catalyzed ACA reaction of other organometallic
reagents to 5,6-dihydro-2H-pyran-2-one, see: (a) Yan, M.; Zhou, Z.-Y.;
Chan, A. S. C. Chem. Commun. 2000, 115. (b) Reetz, M. T.; Gosberg, A.;
Moulin, D. Tetrahedron Lett. 2002, 43, 1189. (c) Brown, M. K.;
Degrado, S. J.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2005, 44, 5306.
(d) Brown, M. K.; Hoveyda, A. H. J. Am. Chem. Soc. 2008, 130, 12904.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 2, 2013
289